Method and structure for producing z-axis interconnection assembly of printed wiring board elements

ABSTRACT

A method of forming a core for and forming a composite wiring board. The core has an electrically conductive coating on at least one face of a dielectric substrate. At least one opening is formed through the substrate extending from one face to the other and through each conductive coating. An electrically conductive material is dispensed in each of the openings extending through the conducting coating. At least a portion of the surface of the conductive coating on one face is removed to allow a nub of the conductive material to extend above the substrate face and any remaining conductive material to thereby form a core that can be electrically joined face-to-face with a second core member or other circuitized structure.

BACKGROUND INFORMATION

[0001] 1. Field of the Invention

[0002] This invention relates generally to a method and structure forproducing a Z-axis interconnection of printed wiring board elements and,more particularly, to a method and structure of providing a printedwiring board formed of a plurality of elements which are laminatedtogether to form a printed wiring board having Z-axis interconnections.

[0003] 2. Background of the Invention

[0004] Printed wiring boards are conventionally made up of a pluralityof individual elements joined together to provide various levels ofwiring on the surfaces of the elements and interconnections between thevarious wiring levels, such interconnection between the various levelsoften being referred to as Z-axis interconnections. In some conventionaltechniques for forming such interconnections in the Z-axis, a drillingoperation is required after the various elements have been joinedtogether. This requires precise alignment of all of the elements as wellas precise drilling of the final structure which creates the possibilityof misalignment at least requiring either rework of the board or at mostscrapping of the board after it reaches this late assembly stage. Thus,it is desirable to provide elements for forming a printed wiring boardand a technique for forming the elements in the printed wiring boardwhich does not require drilling in the final stage but, rather, allowsthe individual elements to be formed with the components of the Z-axisconnection which, when finally joined together, will provide thenecessary connection between various layers of metal wiring.

SUMMARY OF THE INVENTION

[0005] According to the present invention, a method of forming a coremember for a composite wiring board and a method of forming thecomposite wiring board, as well as the core member of the compositewiring board and the wiring board, are provided. The core member isformed by providing a dielectric substrate having opposite faces andforming an electrically conductive coating on at least one face thereof,preferably by laminating copper on the at least one face. At least oneopening is formed through the substrate extending from one face to theother and through each conductive coating. An electrically conductivematerial is dispensed in each of the openings extending through theconducting coating. At least a portion of the surface of the conductivecoating on at least one face is removed to allow a nub of the conductivematerial to extend above the substrate face and any remaining conductivematerial to thereby form a core that can be electrically joinedface-to-face with a second core member or other circuitized structure.In one embodiment, a second core is then formed in a similar manner andthe two cores joined face to face to provide a printed wiring board withelectrical interconnections in the Z-axis, i.e. between the circuittraces on opposite faces of the circuit board so formed. In anotherembodiment, the core is used to join with at least one other circuitizedmember. The invention also contemplates a core member formed accordingto this invention and a printed wiring board formed using at least onecore member.

DESCRIPTION OF THE DRAWINGS

[0006] FIGS. 1-6 show a longitudinal, sectional view, somewhatdiagrammatic, of the steps to form a core member according to oneembodiment of the present invention;

[0007]FIGS. 7 and 8 show the steps of laminating two core memberstogether to form a printed wiring board according to one embodiment ofthe invention;

[0008]FIG. 9 shows an another starting material for a core member;

[0009]FIGS. 10 and 11 show another technique of forming core members forjoining together;

[0010]FIGS. 12 and 13 show two core members formed according to thetechnique shown in FIGS. 10 and 11 joined together;

[0011]FIG. 14 shows another embodiment of a bonding film; and

[0012]FIGS. 15 and 16 show another embodiment of a core joining twocircuitized structures to form a printed wiring board.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Referring now to the drawings and, for the present, to FIGS. 1-6,the successive steps in forming a core member 10 for use in laminatingto another core member to form a printed wiring board according to oneembodiment of the invention are shown. As can be seen in FIG. 1, thecore member 10 includes a dielectric substrate 12 which has metalcoatings 14 and 16 on opposite faces thereof. Dielectric substrate 12can be any conventional dielectric, such as FR4 (a glass reinforcedepoxy), polyimide, polytetrafluroroethylene or other suitable well knowndielectrics. In the embodiment shown in FIGS. 1-6, the metal coatings 14and 16 preferably are copper and, typically, the layer is either oneounce copper (35 um thick) or two ounce copper (70 um thick). However,other thicknesses of copper coatings can be used.

[0014] As shown in FIG. 2, a plurality of holes, two of which are shownat 18, are drilled entirely through the substrate 12 and the metalcoatings 14 and 16. Optionally, the holes 18 may be plated with a metal20, as shown in FIG. 3, to provide a conductive path between thecoatings 14 and 16. However, with good and complete filling ofconductive material, as will be explained presently, the plating 20 maybe partially or fully omitted. Plating 20 can be any type ofconventional plating to provide the openings with such metal, such aselectroless or electrolytic plating.

[0015] As shown in FIG. 4, the openings 18 are filled with a conductivematerial 24. Filling can be done by screening, stenciling, floodcoating, doctor blading, immersing or injecting. In some case, it may bedesirable to heat the adhesive to enhance its flow characteristics and,in some cases, multiple passes might be required to achieve a completefill of the holes 18. Suitable fill materials include conductivepolymers and polymers filled with conductive particles, such as solder,copper particles, silver particles or plated filler particles ormixtures thereof. The polymer can be thermoset or thermoplastic and canbe thinned with a solvent if required. A preferred adhesive material 24is a conductive epoxy sold by the Ablestik Corporation under thetrademark Ablebond 8175 which is a silver filled thermosetting epoxy.Following the filling of the holes as shown in FIG. 4, the epoxy isB-staged, which entails heating the material to a temperature of about130° C. until the degree of cure of the adhesive is advanced to fromabout 20% to about 80% of complete cure. As will become apparent later,the fill material 24 should not be fully cured at this state since itwill be used to adhere to another conductive epoxy in another coreelement to form the printed wiring board, all of which will be describedpresently. If a complete fill of the conductive material results fromthe filling process, the plating 18 may be omitted. However, platinggenerally increases the area of contact, increasing reliability andlowering the electrical contact (resistance) and, therefore, ispreferred.

[0016] It should be noted that if some residue of the adhesive material24 remains on the surface of the conductive layers 14 or 16 afterfilling the holes and after it is B-staged, it can be easily removed bychemical or mechanical polishing processes as are well known in the art.It should be noted that, at this point in the processing, no masks havebeen required for filling the holes so there are no registrationsrequired, and any film material that may accidentally remain on thesurface of the metal can be easily removed by polishing or otheroperations.

[0017] As seen in FIG. 5, layers of material 14 and 16 are partiallyetched away to form thinned or circuitized conductive surfaces 26 and28. Such partial etching can be performed by the technique described inU.S. patent application Ser. No. 08/968,988, filed Nov. 12, 1997, for“Printed Circuit Board with Continuous Connective Bumps” (IBM Docket No.EN9-97-032). Thus, this technique of etching around the protrusions 24by “cold” cupric chloride etching provides the ideal uniform height ofthe fill material 24 above the etched surfaces 26 and 28 on oppositesides of the dielectric material 12. It is important to note that thereare several advantages to the element in the form that it is in, asshown in FIG. 5. The very uniform height of the resulting protrusions ofthe fill material 24 is a particular advantage. Moreover, there is nofill adhesive residue on the metal surface, and no screening operationis required to form a protruding bump of adhesive.

[0018] Following the general selective etching of the surfaces 26 and28, the core is personalized as shown in FIG. 6. In this portion of thepersonalization, the material 26 connects between the protrusions 24 onone face thereof, whereas the material 28 has been entirely removed fromthe other face. This personalization preferably is done by knownphotolithographic processes using either positive or negativephotoresist materials. At this point, the core element as shown in FIG.6 is ready to be laminated with at least one other core element to forma printed wiring structure. It is to be understood that several coreelements typically will be laminated together but the invention is beingillustrated using only two core elements, as shown in FIGS. 7 and 8.

[0019] As can be seen in FIG. 7, two core elements 10 a and 10 b areprovided which are to be laminated together. It will be noted that thetwo core elements 10 a and 10 b are very similar except that thepersonalization on each of them is slightly different in that on thecore element 10 a the personalization 26 a extends from one fillmaterial 24 a to another fill material 24 a, whereas on element 10 b thepersonalization 26 b extends in opposite directions from each of thefill materials 24 b. As seen in FIG. 7, a pre-drilled bonding film 30,such as the film sold under the trademark Pyralux LF by PyraluxCorporation, is interposed between the two cores 10 a and 10 b. The film30 has openings 32 drilled therein which are positioned to align withthe conductive fill material 24 a, 24 b in the two core elements 10 aand 10 b. Heat and pressure are applied as represented by the arrows inFIG. 8 to cause the two core members to bond together, with the PyraluxLF film acting as an adhesive bond material. Also, the fill material 24a and 24 b in each of the openings in the two core members 10 a and 10 bwill bond together, as shown in FIG. 8, to form a continuous Z-axiselectrical connection between the personalization 26 a on the coreelement 10 a and the personalization 26 b on the core element 10 b. Thelamination process also advances the cure of the conductive fillmaterial 24 a and 24 b past 80% to the fully cured stage.(Alternatively, the film material 30 could be a dry film adhesive whichis B-staged and used to laminate the core elements 10 a and 10 btogether.)

[0020]FIG. 9 shows the starting configuration of a slightly differentembodiment for a core element 10 c. In this embodiment, the dielectricmember 12 is coated with a first metal, such as copper 40 on one faceand the same metal 42 on the other face, and a second metal 44 on top ofthe metal 40, and metal 46 on top of the metal 42, which is differentfrom copper. Then, after drilling and optionally plating the drilledholes and filling the holes with conductive material, the metal 44 and46 is selectively etched, leaving the metal 40, and which would providethe same configuration as shown in FIG. 5. The metal 44, 46 could becopper, chromium or copper-invar-copper, as well as others, which can beetched by standard etching processes.

[0021] FIGS. 10-14 show another embodiment or technique for joining twocore elements together to form a printed wiring board. In thistechnique, two cores 10 a and 10 b are provided, but instead of aseparate dry film 30 interposed between the two members to form alaminate structure, a dielectric adhesive material 48 a and 48 b isapplied to the face of at least one, and preferably both, of the coreelements 10 a and 10 b. The adhesive material, which is shown,preferably is a thermoset epoxy such as Epoxy 240 sold by the DexterCorporation. After the epoxy film 48 b is coated onto a face, as shownin FIG. 10, it is removed from the surface of the conductive material 24b, as shown in FIG. 11. This is preferably by a planar polishingtechnique. The same technique is performed on core element 10 a. The twocore elements 10 a and 10 b, with the epoxy films 48 a and 48 b coatedthereon, as shown in FIG. 12, are then laminated together as shown inFIG. 13. The laminate structure is very similar to that shown in FIG. 8with the two epoxy films 48 a and 48 b fusing together as film 48.

[0022] In still another embodiment, the bonding film 30 takes the formof another multilayer structure, such as a conductive sheet of material50 having an adhesive coating, such as an epoxy 52 coated thereon, andB-staged, as shown in FIG. 14. The conductive element 50 preferably hasholes 54 drilled therein to correspond in location to the conductivefill materials 24 a and 24 b so that it can be laminated. The epoxycoating 52 on the conductive element 50 can be eliminated in one or moreopenings 54, thus providing contact with the conductive element 50 bythe conductive material 24 a, 24 b in the cores 10 a or 10 b (not shownin this Figure).

[0023]FIGS. 15 and 16 show yet another technique of forming a coremember 10 and using the core member to join two circuitized structuresto form a printed wiring board. The core member 10 is very similar inconstruction and technique for forming to that shown in FIGS. 1-6,except that, in this embodiment, all of the metal coatings 14 and 16 onboth faces of the substrate are removed so that there is a substrate 12having holes 18 therein, which holes are plated with conductive metal 20just as in the embodiments shown in FIGS. 1-6. The conductive material24 disposed in the holes 18 extends above both faces of the substrate12. This core 10 is then used to join two additional wiring circuitizedstructures 60 and 62. These two wiring structures 60 and 62 are formedin a manner very similar to the formation of the core 10 except that,instead of partially removing the metal coatings 14 and 16 to providethinned coating materials 24 and 26 as with the embodiment shown inFIGS. 1-6, the opposite sides of the core members 60 and 62 arepersonalized so that the conductive epoxy remains at the same height asthe copper coatings 14 and 16. These coatings 14 and 16 are thenpersonalized in the core 60 to provide a circuit trace 64 on one face ofthe core 60 and a pair of bonding pads 66 on the opposite face of thecore 60. On the core 62, the metal coatings 14 and 16 are personalizedon opposite faces thereof to form pads 68 on one face of the substrateand pads 70 on the other face.

[0024] As shown in FIG. 16, the core 10 is disposed between the twocircuitized members 60 and 62, then laminated by heat and pressure as inthe previous embodiment. This will provide a printed wiring boardstructure having continuity between points a and b on one side of theboard through the circuitization 64 on the opposite side of the printedwiring board.

[0025] It is to be understood, of course, that the hole in the core 10and in the circuitized structures 60 and 62 may be formed withoutplating of conductive material thereon, and that the techniques shown inFIGS. 10-13 could be used for joining the core to the circuitizedstructures 60 and 62.

[0026] This invention permits more direct wiring and avoids the need torun additional wiring to a local plated through hole to adjacent layers.Avoiding a plated through hole also permits the connections to bedistributed over the XY plane, eliminating concentration of stress andcracks between the plated through hole and the inner planes.

[0027] A plated through hole tends to be a rigid structure that connectsthe top and bottom of the composite to the internal planes. Thestructure of this invention avoids this and uses materials that aregenerally more compliant. Data has been recorded that shows enhancedreliability during current induced thermal cycles (CITC) testing. Notonly is composite drilling not required as in prior art conventionalboards, desmear (to swell and remove drill waste) and plating to formthe connections between the layers is eliminated.

[0028] While the invention has been described in conjunction withembodiments thereof, it is evident that many alternatives,modifications, and variations will be apparent to those skilled in theart in light of the foregoing teachings. Accordingly, the invention isintended to embrace all such alternatives, modifications and variationsas fall within the spirit and scope of the appended claims.

What is claimed is
 1. A core member for joining to at least oneadditional structure to form a wiring board composite comprising: adielectric substrate having first and second opposite faces and at leastone through opening extending between said opposite faces; electricallyconductive material disposed in each of said openings having a nubextending beyond the opposite faces to thereby form an extendedconductive material for electrically connecting to another.
 2. Theinvention as defined in claim 1 wherein said electrically conductivematerial is an electrically conductive adhesive. The invention asdefined in claim 1 wherein said electrically conductive material is afilled thermoset or thermoplastic polymer.
 4. The invention as definedin claim 4 wherein said electrically conductive material is a filledpolymer.
 5. The invention as defined in claim 2 wherein the polymer iscured to between about 20% and about 80% of complete cure.
 6. Theinvention as defined in claim 1 wherein the material is a filled epoxy.7. The invention as defined in claim 1 wherein said through openingsinclude conductive material plated on the surfaces thereof.
 8. Theinvention as defined in claim 1 wherein conductive traces are formed onat least one face thereof. The invention as defined in claim 8 whereinthe nub of said electrically conductive material extends beyond theconductive traces.
 10. The invention as defined in claim 1 wherein thenub of said electrically conductive material contacts said electricallyconductive traces.
 11. A composite wiring board comprising: at leastfirst and second core members each core member having opposite faces;each core member having at least one through hole each extending betweensaid opposite faces; conductive traces on at least one face of each ofsaid core members; electrically conductive material disposed in each ofsaid openings; and said core members being bonded to each other in faceto face relationship, with the conductive material in each opening insaid first core member being in contact with the conductive material inan opening in said second core member to thereby provide a conductivepath between the non-joined faces of said first and second core members.12. The invention as defined in claim 11 wherein a film of bondingadhesive is disposed between and in contact with the joined faces ofsaid two core members.
 13. The invention as defined in claim 11 whereinsaid electrically conductive material is an electrically conductiveadhesive.
 14. The invention as defined in claim 11 wherein saidelectrically conductive material is a filled thermoset or thermoplasticpolymer.
 15. The invention as defined in claim 11 wherein saidelectrically conductive material is a filled polymer.
 16. The inventionas defined in claim 11 wherein said conductive material is a filledepoxy.
 17. The invention as defined in claim 15 wherein said polymer iscured to greater than 80% of complete cure.
 18. The invention as definedin claim 12 wherein electrically conductive material is deposited on thesurface of said openings.
 19. The invention as defined in claim 12wherein the conductive traces are on at least the non-joined faces ofsaid two core members and contact the conductive material in saidopening.
 20. A method of forming a core member for joining to at leastone additional core member to form a composite comprising the steps of:providing a dielectric substrate having opposite faces; forming anelectrically conductive coating on at least one face thereof; forming atleast one opening through said substrate extending from one face to theother and through each conductive coating; dispensing an electricallyconductive material in each of said openings extending through eachconductive coating; removing at least a portion of surface of theconductive coating on at least one face to thereby allow a nub of theconductive material to extend above the surface of said substrate tothereby form a core than can be electrically joined face to face withanother structure through said conductive material.
 21. The invention asdefined in claim 20 wherein said electrically conductive material is anelectrically conductive adhesive.
 22. The invention as defined in claim20 wherein said electrically conductive material is a filled thermosetor thermoplastic polymer.
 23. The invention as defined in claim 20wherein said electrically conductive material is a filled polymer. 24.The invention as defined in claim 20 wherein the conductive material isa filled epoxy.
 25. The invention as defined in claim 23 wherein thepolymer is cured to between about 20% and about 80% of complete cure.26. The invention as defined in claim 20 further including the step ofplating a conductive material on the wall of each of said opening beforedispensing said conductive material in each of said openings.
 27. Theinvention as defined in claim 20 wherein the electrical conductivecoating is a metal, and the portion of the surface is removed by partialetching.
 28. The invention as defined in claim 20 wherein the electricalconductive coating is formed as two layers of different metals and wherethe removal of the surface portion of the electrical conductivitycoating is removed by differential etching.
 29. The invention as definedin claim 20 wherein at least one circuit trace is formed from saidconductive coating and the nub of said conductive material extends abovesaid at least one circuit trace.
 30. A method of forming a compositewiring board comprising the steps of: forming first and second coremembers according to claim 20; and joining said first and second coremembers in face-to-face relationship with the conductive material ineach opening in said first core member contacting the conductivematerial in one of said openings in said core member.
 31. The inventionas defined in claim 30 wherein said first and second core members arebonded together with a bonding adhesive film interposed therebetween.32. The invention as defined in claim 30 wherein said electricallyconductive material is an electrically conductive adhesive.
 33. Theinvention as defined in claim 30 wherein said electrically conductivematerial is a filled thermoset or thermoplastic polymer.
 34. Theinvention as defined in claim 30 wherein said electrically conductivematerial is a filled polymer.
 35. The invention as defined in claim 30wherein said conductive material is a filled epoxy.
 36. The invention asdefined in claim 35 wherein said polymer is cured to more than about 80%of complete cure.
 37. The invention as defined in claim 31 wherein saidadhesive is provided as a thin film on at least one of said coremembers.
 38. The invention as defined in claim 31 wherein said adhesiveis provided as a circuit structure having adhesive coatings on eachside.
 39. A composite wiring board comprising: a core member havingopposite faces; said core member having at least one through hole eachextending between said opposite faces; electrically conductive materialdisposed in each of said openings and extending beyond each facethereof; and said core member being bonded to at least one additionalwiring structure having bonding pads on one face thereof, with theconductive material in each opening in said core member being in contactwith a bonding pad on said additional wiring structure to therebyprovide a conductive path between the non-joined faces of said coremember and said additional structure.
 40. The invention as defined inclaim 39 wherein said electrically conductive material is anelectrically conductive adhesive.
 41. The invention as defined in claim39 wherein said electrically conductive material is a filled thermosetor thermoplastic polymer.
 42. The invention as defined in claim 39wherein said electrically conductive material is a filled polymer. 43.The invention as defined in claim 39 wherein said conductive material isa filled epoxy.
 44. The invention as defined in claim 42 wherein saidpolymer is cured to greater than 80% of complete cure.
 45. The inventionas defined in claim 39 wherein electrically conductive material isdeposited on the surface of said openings.
 46. The invention as definedin claim 39 wherein conductive traces are formed on at least thenon-joined faces of said two core members and contact the conductivematerial in said opening.
 47. A method of forming a composited wiringboard comprising the steps of: forming a core member according to claim20; and joining said core member in face-to-face relationship withanother circuitized structure having bonding pads on one surfacethereof, with the conductive material in each opening in said coremember contacting a bonding pad.
 48. The invention as defined in claim47 wherein said electrically conductive material is an electricallyconductive adhesive.
 49. The invention as defined in claim 47 whereinsaid electrically conductive material is a filled thermoset orthermoplastic polymer.
 50. The invention as defined in claim 47 whereinsaid electrically conductive material is a filled polymer.
 51. Theinvention as defined in claim 47 wherein said conductive adhesive is afilled epoxy.
 52. The invention as defined in claim 50 wherein saidpolymer is cured to more than about 80% of complete cure.
 53. Theinvention as defined in claim 12 wherein said film of bonding adhesiveis a dry film of material having openings formed therein.
 54. Theinvention as defined in claim 12 wherein said film of bonding adhesiveis an adhesive coated conductive material.
 55. The invention as definedin claim 31 wherein said film of bonding adhesive film is a dry film ofmaterial having openings formed therein.
 56. The invention as defined inclaim 31 wherein said film of bonding adhesive is an adhesive coatedconductive material.